Hot filament chemical vapor deposition (HFCVD) has been extensively used by researchers to deposit polycrystalline diamond on a variety of substrates. The technique and reactor designs typically used for HFCVD of diamond are described in detail in an article entitled "Growth Of Diamond Particles From Methane-Hydrogen Gas" published in J. Materials Science 17, 3106(1982) by Matusumoto et al. Since this disclosure, numerous researchers have attempted to improve the HFCVD technique. This development can be found in the review article by C. E. Spear entitled "Diamond-ceramic coating of the future" published in J. Am. Ceram. Soc. 72(2), 171(1989). The reactor generally comprises a resistively heated filament and a heated or cooled substrate stage which are housed in a reactor chamber with pumping and pressure monitoring facilities. The filament is made from a high melting-point refractory metal that is used to dissociate hydrogen and other molecules in a feed gas which normally contains a mixture of hydrogen and hydrocarbon. Atomic hydrogen and other dissociated products subsequently react with the feed gas to generate precursors responsible for diamond formation. The precursors then diffuse to and condense on the substrate for the formation of polycrystalline diamond. The separation between the filament and the substrate is normally in the range 0.5 to 5 cm. With this small distance, a sufficient amount of growth precursors diffuses to the substrate prior to their recombination into more stable molecules.
A major advantage of HFCVD of diamond films, relative to other methods of diamond film growth such as microwave plasma CVD (MWCVD), radiofrequency CVD, and plasma jet CVD, is the low equipment investment costs, and the ease in scaling up the production to a large area substrate. The diamond growth rate using HFCVD does not normally exceed 5 .mu.m/hr and is typically about 1 .mu.m/hr (see e.g., International PCT Patent Publication WO 91/14798, by Garg, et al., entitled "An Improved Hot Filament Chemical Vapor Deposition Reactor"), which is not high enough for economically viable thick film production. A major disadvantage of HFCVD, as with other known diamond growth methods, is that it requires scratching or diamond-seeding of the substrate surface to initiate diamond nucleation. Such a pretreatment induces a high defect concentration on the substrate surface and thus generally precludes the possibility of obtaining heteroepitaxial growth of diamond. This pretreatment increases the CVD diamond production costs.
A method of achieving nucleation enhancement is disclosed by Yugo et al. in an article entitled "Generation Of Diamond Nuclei By Electric Field In Plasma Chemical Vapor Deposition" and published in Applied Physics Letters 58(10), 1036-1038(1991) which proposes a predeposition of diamond nuclei on a silicon mirror surface prior to the conventional diamond CVD growth process. Yugo et al. reported that diamond nuclei growth required a high methane content in hydrogen and did not occur below 5%, and that high densities of nuclei occurred only above 10% methane. Yugo et al. also reported that the substrate bias against the CVD plasma should be below 200 volts to avoid sputtering and the typical bias was 70 volts. The total time duration for the pretreatment was limited to between 2 to 15 minutes.
More recently, Stoner et al. (see e.g., World Patent # 93/13242, entitled "Nucleation Enhancement For Chemical Vapor Deposition Of Diamond") and Jiang et al. (see e.g., "Epitaxial Diamond Thin Films On (001) Silicon Substrates", Applied Physics Letters 62(26), 3438-3440(1993)) have independently disclosed diamond nucleation enhancement by negatively biasing the substrate against the CVD plasma during MWCVD of diamond films on silicon. More importantly, both of these groups showed that the preservation of the crystallinity of the silicon substrate surface as a result of the elimination of any scratching/diamond-seeding pretreatment, together with the nucleation enhancement, allows the heteroepitaxial formation of diamond (100) nuclei on Si (100). In the method described by Jiang, et al., the substrate was biased at -100 to -300 V relative to the microwave plasma with a typical recipe for MWCVD of diamond using CH.sub.4 /H.sub.2. In the method described by Stoner et al., the negative bias of the substrate required for nucleation enhancement was claimed to be not less than 250 volts. The nucleation of diamond and heteroepitaxial nucleation of diamond with a modified HFCVD-DC plasma method and apparatus, which require much less equipment investment than the MWCVD approach, is one advantage of the present invention discussed hereinafter.
Modifications of the conventional HFCVD by coupling it with DC plasma CVD have been proposed previously by A. Ikegaya and T. Masaaki in JP 173366 (1986), JP 75282 (1987), and European Patent Publication 0254312 A1. In this approach, a hot filament array is used as a thermionic electron emitter and a grid electrode is inserted between the hot filament array and the substrate. The filament array and the substrate are both negatively biased against the grid electrode in order to form two DC plasma zones, one between the filament array and the grid electrode, and the other between the grid electrode and substrate. In these two plasma zones, the plasma density in the grid-filament zone is much higher than that between the grid-substrate zone because of thermionic electron emission from the hot filaments. In the grid-filament zone, ions are extracted towards the filaments, i.e., further away from the substrate. Through gas phase collision, this extraction will also move the reactants generated near the filament away, instead of towards, the substrate. Ikegaya et al. reported a growth rate of 2 .mu.m/hr on a tungsten carbide substrate using 1% methane in hydrogen, a power density of 40 W/cm.sup.2 between the hot filament and grid and 20 W/cm.sup.2 between the grid and substrate with a hot filament temperature of about 2000.degree. C. and a substrate temperature of 980-1010.degree. C. and a pressure of 90 torr. A growth rate of 12.5 .mu.m/hr was also reported for a gas mixture of 2% (CH.sub.3.sub.2 CN in H.sub.2 with a power density of 60 W/cm.sup.2 between the filament and grid, and a power density of 40 W/cm.sup.2 between the grid and substrate. Ikegaya et al. noted that a DC plasma power density higher than 200 W/cm.sup.2 between the grid and substrate led to sputter-etching of the substrate. Ikegaya et al. reported that this problem arises because the negative bias on the substrate against the grid attracts ions to the substrate. A high DC plasma power density results in a high bombardment energy and high current density, and the induced energetic particle bombardment causes detrimental sputter-etching.
A logical approach for eliminating the sputtering problem is to connect the substrate to the grid or simply to discard the grid. A DC plasma can still be maintained by biasing the filament negatively against the substrate. In fact, A. Ikegaya and N. Fujimori showed such a configuration in a JP 176762 and a PCT Patent Publication WO92/01828. However, a drawback to both of these designs is that they do not allow for any ion extraction towards the substrate during nucleation and growth.
Thus, there still exists a need to modify the HFCVD method and apparatus in order to provide an economical approach to control energetic particle bombardment for improved diamond nucleation and growth.